I used to build AMD systems up until socket 939 was phased out in favor of socket AM2. I then switched to Intel socket 775 systems for the next 4 years. My neice still has my older socket 462 Athlon 2.2GHz system and is using it to play games, chat, do homework. This system is 10 years old now.
The motherboard was an ASUS A7N8X-VM with onboard video. Probably the best motherboard I ever bought for longevity. I had a lot of MSI and ECS boards which died in 1-2 years.
On the Intel side of things, 3 boards, 1 ASUS running a 2.4GHz Dual Core Pentium at 3GHz for 3 years solid, and an ASROCK motherboard Running a 1.8GHz Core2Duo 4300 for 3 years at 2.4 Ghz. The ASROCK was an RMA replacement for the previous 4CoreDual-VSTA which ran only just under 1 year. These 2 systems are still currently in use at my house.
My newest motherboard is just a month old it is a GigaByte GA-MA785GM-US2H running a AMD Phenom II 965BE 3.4 GHz processor overclocked to 4GHz on a straight mutliplier overclock. I hope I get a lot of life out of it...
So it would seem that longevity-wise, the manufacturer of the Motherboard is far more important than who makes the processor. I even ran a Cyrix processor (remember those?) for a number of years on an ASUS motherboard. My best results so far have been on ASUS and GigaByte motherboards, and they will remain the 2 companies I will buy from for some time, regardless of processor brand...